A ferroelectric material is characterized by the presence of spontaneous polarization, even when the material is not in an electric field. This phenomenon arises from the presence of permanently-polarized dielectrics forming lines parallel or antiparallel within the material. In addition, the direction of polarization can be reversed by application of an external electric field. By taking advantage of this property, a ferroelectric material can be applied to various electronic components, such as a pyroelectric infrared detecting element, a piezoelectric element, a light modulator using electro-optical characteristics, or to a non-volatile memory element. A typical example of a well-known ferroelectric material is a perovskite crystal structure oxide, e.g., PbTiO.sub.3, Pb.sub.1-X La.sub.X Ti.sub.1-X/4 O.sub.3 (PLT), PbZrXTi.sub.1-X O.sub.3 (PZT), BaTiO.sub.3. Among these oxides, a PbTiO.sub.3 type ferroelectric material is thought to be a promising pyroelectric material because of its high Curie temperature, large pyroelectric coefficient, preferably small dielectric constant, and small dielectric loss. This material is already put to practical use in the form of an infrared sensor using ceramics.
Merits of this pyroelectric infrared sensor are that the operation can take place at room temperature, and the sensor has no wavelength dependency. Besides, the pyroelectric infrared sensor is superior among thermal type infrared sensors with respect to sensitivity and response speed.
At present, most ferroelectric materials used for an infrared detecting element or a piezoelectric element are polycrystalline ceramics. Along with the recent tendency of electronic components towards compact size, smaller electronic components compatible with ferroelectric materials are also demanded. Furthermore, since a pyroelectric element is formed thinner, heat capacity decreases and sensitivity increases accordingly. Therefore, due to the need for improved performance of an infrared detecting element, and also because of the development towards smaller and lighter components as mentioned above, the formation of a ferroelectric single crystal thin film which can achieve high sensitivity and high speed response has been drawing attention.
For example, a pyroelectric infrared sensor using a PbTiO.sub.3 type thin film of c-axis orientation is reported in J. Appl. Phys., Vol. 61, P. 411 (1987). Also, a ferroelectric thin film is disclosed in Laid-open Japanese Patent Application No. (Tokkai Sho) 59-138004 which discloses improved performance index by adding a small amount of La.sub.2 O.sub.3 to PbTiO.sub.3. Furthermore, as disclosed in Laid-open Japanese Patent Application No. (Tokkai Sho) 59-141427, a small amount of MnO.sub.2 is added to PbTiO.sub.3 for improving figure of merit and dielectric loss of a ferroelectric thin film. In addition, Laid-open Japanese Patent Application No. (Tokkai Sho) 61-88403 discloses a mono phase ferroelectric PbTiO.sub.3 thin film having a high electro-optical effect by selecting a Pb/Ti molar ratio in the PbTiO.sub.3, and also having pyroelectricity and piezoelectricity. Also, it is disclosed in Laid-open Japanese Patent Application No. (Tokkai Hei) 3-245406 that a small amount of MgO is added to PbTiO.sub.3 for obtaining a ferroelectric thin film having a high direct current resistivity and a high pyroelectric coefficient.
In the various conventional techniques mentioned above for forming thin films by using PbTiO.sub.3 type materials, there has been some improvement made regarding pyroelectric characteristics or piezoelectric characteristics as well as with regard to resistivity, withstand voltage, and dielectric loss. However, a thin film which has satisfactory characteristics in all respects is not yet obtained.